Amorphous SiO 2 fracture surfaces created under different partial pressures of water vapor (p H 2 O ) were analyzed using temperature-programmed static secondary ion mass spectroscopy. The results were used to develop an atomistic model for the formation of a fracture surface. It was found that substantial reconstruction of the SiO 2 fracture surface took place immediately after the fracture event. Formation of the fracture surface was modeled as three individual steps-rupture of Si-O-Si bonds to form dangling Si* and Si-O* bonds, reconstruction and relaxation of the surface to form both strained and unstrained siloxane bonds, and, lastly, reaction of H 2 O molecules with strained siloxane bonds to form surface silanol groups. The final concentration of surface silanol groups was found to have only a weak dependence on the p H 2 O in the ambient atmosphere during the fracture process. It was also found that the number of strained siloxane bonds on the SiO 2 fracture surface could be substantially reduced by heat treatment of the glass under vacuum.

The hydroxylation and dehydroxylation behavior of amorphous silica fracture surfaces was studied using temperatureprogrammed static SIMS. The results show that vacuum heat treatments result in more extensive condensation of silanol groups on the silica glass fracture surface as compared to fumed silica (Cabosil). This is attributed to differences in the distribution of silanol groups on the two silica surfaces. The rehydration kinetics of the dehydroxylated silica fracture surfaces showed two distinct reaction rates-an initial rapid increase in the silanol concentration, followed by a slower rehydration for longer dosing times. The slower rehydration reaction was shown to follow first-order reaction kinetics with the reaction rate constant, suggesting hydrolysis of strained siloxane bonds on three-membered silicate ring structures. The much faster initial rehydration is attributed to the hydrolysis of extremely strained siloxane bonds in two-membered, edgeshared tetrahedral rings. The effect of the dehydration time and temperature (i.e., thermal history of the surface) on the rehydration kinetics is also discussed.